1. Technical Field of the Invention
The present invention relates to a focusing controller for controlling the focusing position of a light beam irradiated onto a multilayer recording medium having a plurality of recording layers.
2. Description of Background Art
In recent years, extensive efforts are being made to develop a multilayer optical disk that is capable of increasing recording capacity per one side as a recording medium of large capacity. Such a multilayer optical disk has a structure in which a plurality of reflecting recording layers are laminated. The multilayer optical disk will be explained by way of example of a dual layer DVD (digital versatile disk) having two recording layers. These recording layers have such a structure that two recording layers are spaced apart from each other via a spacer layer, so that a relatively small distance exists therebetween. These recording layers are called a first recording layer (hereinafter simply called a first layer) or a layer 0 (L0), and a second recording layer (hereinafter simply called a second layer) or a layer 1 (L1) in order from a disk surface (i.e., from the layer that is nearest to an optical pickup).
In a recording or reproducing operation of the dual layer disk, a laser beam is focused onto any recording layer, and a signal is recorded to this recording layer or the signal is reproduced from this recording layer. Furthermore, in association with the recording or reproduction operation there is a case in which the focusing position of the irradiating light beam is changed from a certain recording layer to another recording layer. Such a movement of the focusing position of the irradiating light beam between the recording layers is generally called a focus jump. For example, a focus jump is disclosed in a technical paper titled xe2x80x9cOutline of Dual layer diskxe2x80x9d of Kuribayashi, et al. printed in Vol. 6, No. 2 (1996), pp. 67-73 of the Journal of Pioneer RandD of Pioneer Co., Ltd.
A conventional method of the focus jump which is generally adopted will next be explained briefly with reference to the drawings.
FIG. 1 typically shows changes in a focus actuator driving electric current and a focus error signal when the focus jump takes place from the first layer (L0) to the second layer (L1). A focus actuator moves an objective lens arranged within the optical pickup in its focusing direction. For example, the focus error signal is obtained by an astigmatism method.
A focus servo is operated in a focusing state onto the L0 layer. In the focus jump, a servo loop is turned xe2x80x9coffxe2x80x9d at a time t0, and the objective lens is simultaneously moved toward the L1 layer by applying an acceleration pulse to the actuator for a constant period (TA). When a certain recording layer is generally focused, the focus error signal shows an S-shape characteristic with respect to the shift of the focusing position. Namely, a focus error signal of the S-shape is obtained about a center at the focus position. It is necessary to move the objective lens in its focusing direction in an opening state of the focus servo loop so as to measure these characteristics of the S-shape. Accordingly, for example, in the case of the dual layer disk, two S-shape characteristics are obtained, and it is too complex to separately measure levels of these two characteristics.
The respective focus error signal characteristics of the L0 and L1 layers are synthesized in a waveform of the focus error signal obtained in the focus jump of the dual layer disk. Accordingly, as shown in FIG. 1, the focus error signal of the S-shape is obtained as the focusing position is moved from one recording layer to another.
Next, one example of the focus jump will be explained with reference to FIG. 1. When the focus jump is instructed, the servo loop is first turned xe2x80x9coffxe2x80x9d, and the acceleration pulse is applied. Thus, a focal point is separated from the L0 layer, and passes through a peak position. When the focus error signal level reaches a predetermined threshold value (xcex94V1) (at a time t1), the application of the acceleration pulse is stopped. Namely, the acceleration pulse (TA) is applied. The focus error signal of the L1 layer appears as the focal point approaches the L1 layer. When the focus error signal level reaches a predetermined threshold value (xcex94V2) (at a time t2), a deceleration pulse having a polarity reverse to the polarity of the acceleration pulse is applied to the actuator. Thus, the objective lens is decelerated. Further, when the focus error signal level reaches a predetermined threshold value (xcex94V3) (at a time t3), the application of the deceleration pulse is stopped. Namely, the deceleration pulse (TD) is applied from t2 to t3. Thereafter, the servo loop is set to the xe2x80x9conxe2x80x9d state. In this way, the focus jump from the L0 layer to the L1 layer is executed.
However, when the focusing control system operates with a multilayer disk having a different focus error signal characteristic in each of recording layers, there is a possibility that the focus jump process shown in FIG. 1 does not work properly. For example, as shown in FIG. 2, when the respective focus error signal levels S0, S1 of the L0 and L1 layers are greatly different from each other (S0 less than S1), a deceleration pulse generating period (TDxe2x80x2) is prolonged in comparison with the case of FIG. 1. In the worst case, there is a possibility that the focus jump fails because no focusing position reaches a target layer (L1 layer). Otherwise, a converging time in the servo control is prolonged because of a deviation of the deceleration pulse from an optimum deceleration pulse.
The present invention has been made in view of the points described above, and an object of the present invention is to provide a simple focusing controller of high performance that performs control operations in consideration of the characteristic of each disk.
A focusing controller according to the present invention is a controller that performs focusing control of a light beam irradiated to a multilayer recording medium having a plurality of recording layers, and comprising an optical pickup including an objective lens for converging the light beam and a detector for receiving reflected light from the multilayer recording medium; an error signal generator for generating a focus error signal of the irradiated light beam from a detecting signal of the detector; an actuator for operating the objective lens; and a controller for controlling a focusing position of the light beam by operating the actuator based on the focus error signal; wherein the controller calculates intensities of the reflected lights of two layers based on the detecting signal of the detector, and thus controls the timing of a control signal for a focus jump between the two layers.